1. Field of the Invention
The present invention relates to an aqueous composition comprising at least one finely divided filler and at least one water-soluble amphoteric copolymer, a process for the preparation of such a composition and its use as an additive in the production of paper, cardboard and board.
2. Description of the Related Art
Polymers having a large number of ionically dissociable groups in the main chain and/or in a side chain are referred to as polyelectrolytes. If these polymers have both anionogenic/anionic and cationogenic/cationic groups, they are amphoteric polyelectrolytes or ampholytic polymers, respectively. They are generally water-soluble or at least water-dispersible and have a wide range of applications, inter alia in the area of papermaking.
EP-A-0 251 182 discloses a process for the preparation of polymers, a mixture of N-vinylformamide and acrylonitrile or methacrylonitrile being polymerized in the presence of free radical initiators, and the polymers then being modified by treatment with acids. The modified polymers are said to contain vinylamine units in the form of salts, vinylformamide units and acrylonitrile or methacrylonitrile units and, if appropriate, acrylamide units and acrylic acid units. The reworking of examples of this publication has, however, shown that the polymers hydrolyzed with acids contain considerable amounts of amidine units of the formula

The hydrolyzed polymers are used as drainage aids, retention aids or for strengthening the paper in papermaking.
EP-A-0 528 409 discloses cationic copolymers which contain from 20 to 90 mol % of amidine units. They are prepared by copolymerization of N-vinylformamide and acrylonitrile and subsequent hydrolysis of the copolymers with acids. The polymers containing amidine units are used as flocculants for sludges.
WO 94/13882 relates to the use of copolymers which are obtainable by copolymerization of N-vinylcarboxamides, monoethylenically unsaturated carboxylic acids and, if appropriate, vinyl acetate, N-vinylpyrrolidone and/or N-vinylimidazole and, if appropriate, monomers having at least two double bonds in the molecule and subsequent partial or complete hydrolysis of the vinylcarboxamide units contained in the copolymers to amino or ammonium groups as an additive for the paper stock for increasing the drainage rate and the retention and the dry and wet strength of the paper in papermaking. As analyses have shown, hydrolyzed copolymers of N-vinylformamide and acrylic acid may contain considerable amounts of amidine units of the following formula

where X− is an anion.
JP-A-08059740 discloses that amphoteric water-soluble polymers are added to aqueous suspensions of inorganic particles, at least a part of the polymers being adsorbed onto the filler surface. The amphoteric polymers are preferably prepared by hydrolysis of copolymers of N-vinylformamide, acrylonitrile and acrylic acid in the presence of acids. They contain from 20 to 90 mol % of amidine units of the structure

where R1 and R2 are each H or a methyl group and X− is an anion. The filler slurries treated with such polymers are added to the paper stock in the production of filler-containing papers. The filler treatment is said to lead to improved drainage of the paper stock and result in an improvement in the various strength properties of the dried paper and an improvement in the filler retention.
US-A-2002/0088579 describes the pretreatment of inorganic fillers with cationic, anionic and amphoteric (zwitterionic) polymers. The treatment consists in each case of at least two stages. First the treatment with a cationic polymer and then the treatment with an anionic polymer are recommended. In further steps, further cationic and anionic polymers can be adsorbed again alternately. The aqueous suspensions containing the pretreated filler particles are added to the paper stock in the production of filler-containing paper. The filler treatment is said to lead to an improvement in various strength properties of the dried paper.
WO 00/59965 describes a polymer composition based on polymers having phosphonate and sulfonate groups and the use thereof in papermaking. The polymers may additionally contain monomers having amide groups incorporated in the form of polymerized units. The use of copolymers which contain N-vinylcarboxamides incorporated in the form of polymerized units and are subjected to a hydrolysis after the polymerization is not disclosed. The use of aqueous slurries of finely divided fillers which are at least partly coated with such polymers for papermaking is also not described.
Unpublished German Patent Application P 103 22 266.9 describes aqueous slurries of finely divided fillers which are at least partly coated with polymers and which are obtainable by treating aqueous slurries of finely divided fillers with at least one water-soluble amphoteric copolymer which is obtainable by copolymerization of
a) at least one N-vinylcarboxamide of the formula

where R1 and R2 are H or C1- to C6-alkyl,
b) at least one monoethylenically unsaturated carboxylic acid having 3 to 8 carbon atoms in the molecule and/or the alkali metal, alkaline earth metal or ammonium salts thereof and, if appropriate,
c) other monoethylenically unsaturated monomers which are free of nitrile groups and, if appropriate,
d) compounds which have at least two ethylenically unsaturated double bonds in the molecule,
and subsequent partial or complete elimination of the —CO—R1 groups from the monomers III incorporated in the form of polymerized units in the copolymer.